1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display (OELD) device and a method and an apparatus of manufacturing the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, organic electroluminescent display (OELD) devices are self-luminescent display devices. The OELD devices operate at low voltages and have a thin profile. Further, the OELD devices have fast response time, high brightness, and wide viewing angles.
FIG. 1 is a circuit diagram illustrating an OELD device according to the related art.
Referring to FIG. 1, the OELD device includes a gate line GL and a data line DL crossing each other to define a pixel region P.
In the pixel region P, a switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst and an organic electroluminescent diode E are formed. A gate electrode and a source electrode of the switching thin film transistor Ts are connected to the gate line GL and the data line DL, respectively. A gate electrode of the driving thin film transistor Td is connected to the drain electrode of the switching thin film transistor Ts. Both electrodes of the storage capacitor Cst are connected to the gate electrode and a source electrode of the driving thin film transistor Td, respectively. The source electrode of the driving thin film transistor Td is connected to a power line PL. An anode of the organic electroluminescent diode E is connected to the drain electrode of the driving thin film transistor Td, and a cathode of the organic electroluminescent diode E is grounded.
When an ON gate voltage is applied to the gate line GL, the switching thin film transistor Ts is turned on, and a data voltage is applied to the data line D. The data voltage passes through the turned-on switching thin film transistor Ts and is applied to the gate electrode of the driving thin film transistor Td. A current passing through the driving thin film transistor Td is adjusted according to the data voltage applied to the driving thin film transistor Td, and the current flows on the organic electroluminescent diode E. The storage capacitor Cst stores the data voltage applied to the driving thin film transistor while the switching thin film transistor Ts is turned off.
The switching and driving thin film transistors Ts and Td and the organic electroluminescent diode E are usually formed on the same substrate of the OELD device. However, forming the thin film transistors and the organic electroluminescent diode on different substrates has recently researched and developed. This type of OELD device is referred to as a dual plate type OELD device.
FIG. 2A is a cross-sectional view illustrating a dual plate type OELD device according to the related art.
Referring to FIG. 2A, the OELD device 1 includes an array substrate and an opposing substrate facing each other. The OELD device 1 includes a display region AA to display images and a non-display region NAA.
The array substrate includes a gate line (not shown) and a data line DL formed on a first substrate 5 and crossing each other to define a pixel region P in the display region AA. A switching thin film transistor (not shown) and a driving thin film transistor Td are formed on the first substrate 5. The data line DL is formed in a data region D, and the driving thin film transistor Td is formed in a driving region Dr.
The driving thin film transistor Td includes a gate electrode 25 connected to the gate line, a semiconductor layer 42, a source electrode 32 and a drain electrode 34. A gate insulating layer 45 is on the gate electrode 25 and the gate line. The semiconductor layer 42 includes an active layer 40 made of intrinsic amorphous silicon and an ohmic contact layer 41 made of impurity-doped amorphous silicon.
A passivation layer 55 is formed on the driving thin film transistor Td and has a drain contact hole DCH exposing the drain electrode 34. A connection electrode 70 is connected to the drain electrode 34 through the drain contact hole DCH.
The opposing substrate includes an organic electroluminescent diode E formed on a second substrate 10. The organic electroluminescent diode E includes a first electrode 80, an organic emitting layer 82 and a second electrode 84.
An auxiliary electrode 60 is on the second substrate 10. The first electrode 80 is on the auxiliary electrode 60. A buffer layer 62 is on the first electrode 80. A separator 64 and a patterned spacer 50 are formed on the buffer layer 62.
The separator 64 has a tapered shape, and the patterned spacer 50 has a reverse-tapered shape. The organic emitting layer 82 and the second electrode 84 are formed on the first electrode 80 and the patterned spacer 50 in the pixel region P.
A portion of the second electrode 84 on the patterned spacer 50 contacts the connection electrode 70 so that the organic electroluminescent diode E on the second substrate 10 is electrically connected to the driving thin film transistor Td on the first substrate 5.
A seal pattern 90 is formed along a peripheral region of the array substrate and the opposing substrate to attach the array substrate and the opposing substrate. Further, the seal pattern 90 functions to keep a cell gap of the OELD device i.e, a gap between the array substrate and the opposing substrate. A space between the array substrate and the opposing substrate is under vacuum.
FIG. 2B is a cross-sectional view illustrating another OELD device according to the related art. The OELD device of FIG. 2B is similar to that of FIG. 2A. Explanations of parts similar to parts of FIG. 2A are omitted.
Referring to FIG. 2B, the OELD device 1 does not include the connection electrode and the patterned spacer (70 and 50 of FIG. 2A). A conductive spacer 52 connects a drain electrode 34 and an organic electroluminescent diode E.
By the sealant 90 of each of FIGS. 2A and 2B along the peripheral portion, the space between the array substrate and the opposing substrate is sealed from the outside. Accordingly, the space between the array substrate and the opposing substrate is empty under vacuum. However, the OELD device having the empty space is weak to exterior impact or pressure, and contact defects between the organic electroluminescent diode E and the driving thin film transistor Td are thus frequently caused. Accordingly, display quality and production efficiency are reduced.
Further, in experiment that the OELD device 1 is under conditions of about 80° C. and humidity of about 95%, moisture permeates inside the space through interfaces between the sealant 90 and the substrates 5 and 10, and lighting defects thus occur about 200 hours later. Accordingly, it is difficult for the OELD device 1 to obtain long lifetime.
To resolve this problem due to the moisture permeation, forming a passivation layer or a metal layer of calcium (Ca), magnesium (Mg) or aluminum (Al) on the second electrode 84 is suggested. However, the passivation layer or the metal layer does not completely cover the organic electroluminescent diode E. For example, a portion of the organic electroluminescent diode E near the separator 64 is not covered by the passivation layer or the metal layer. Accordingly, the moisture permeates into the not-covered portion of the organic electroluminescent diode E, and defects thus occur.
Further, in the process of forming the passivation layer or the metal layer, impurities of several micrometers are frequently stuck to the opposing substrate. Portions where the impurities are stuck become dark spots, and display quality and production efficiency are reduced.
Further, as the OELD increases in size, due to a difference of light exposure amount between a center portion and a peripheral portion of the OELD device, it becomes more difficult to form the patterned spacers so as to make thickness uniform over the entire plane of the OELD device. Accordingly, contact defects between the organic electroluminescent diode and the driving thin film transistor in some pixels are caused, and thus display quality and production efficiency are reduced.